Conventional vertical-double diffusion MOSFETs with a large current capacity for power use, as shown in FIG. 3, and IGBTs taking advantage of the conductivity modulation via a carrier injection, as shown in FIG. 4, have a structure in which a p-type base region (6) and a n+-type source region (8) both formed on the surface side of an n-type drain layer through a double diffusion process. The methods used to manufacture these devices have many points in common.
The conventional manufacturing method for these structures, which are shown in FIG. 3 and 4, includes first forming an n+-type drain layer (1) on an n+-type layer (2) that has been formed on a p+-type silicon substrate (10). Next a p+-type contact region (3) is formed on the surface of this drain layer (1), which is covered with a gate oxide film (41), and gate electrodes (5) are formed on the gate oxide film (41). After removing the gate oxide film (41) around the gate electrodes (5), the p-type base region (6) is formed by diffusion using a self-aligning process using the gate electrodes as a mask, while, in the case of the IGBT, a low p+-type resistance region (7) is further formed in the base region (6). Thereafter, using the gate electrodes (5) again as a mask, the n+-type source regions (8) are formed, their surfaces are covered with oxide films (42), and the surfaces of the contact region and source regions are opened to form a source electrode (11). A drain electrode (12) is formed on the back surface of the silicon substrate (2) and the other silicon substrate (10) have a drain electrode (12).
The above-described conventional manufacturing methods have the following problems. Specially, in switching elements that have a MOS structure represented by a vertical-double diffusion MOSFET and an IGBT, it is very important to raise the quality of gate insulation films, and suppress crystalline defects in the semiconductor layer immediately below the gate oxide films in order to improve and stabilize the element characteristics. However, in the above manufacturing methods, in which highly concentrated B ions are introduced through an ion-injection process, and then heat treatment is carried out to form the contact region (3), injection damage resulting therefrom causes crystalline defects on the surface of the drain layer (1) and around its vicinity. The quality of gate oxide films is therefore degraded and many crystalline defects remain in the region immediately below the gate oxide films. As a result, break-downs of the gate oxide films, short circuits between the gate and the drain, and other failures develop frequently, thereby reducing the element yield.
Alternatively, a method is used wherein the surface of the drain layer (1) is initially covered with an oxide film with a thickness of approximately 500 A, through which an ion injection is introduced to prevent contamination of the surface of the drain layer (1) and ion damage; however, this method requires an extra process to form the oxide film.
A further method is available in which a highly concentrated defective region is formed intentionally on the rear side of the drain layer (1) through an ion injection process or the like to reduce the contaminants and damage on the surface of the drain layer (1). This method, however, requires several separate processes.
It is an object of the present invention to solve these problems by suppressing the generation of contamination and defects on the gate oxide films, and in the region immediately below the films, while improving and stabilizing the element characteristics, and enhancing the element yield by forming respective regions in the drain layer with the surface of the drain layer (1) covered, without increasing the number of production processes.